From thorite to coffinite: a spectroscopic study of Th(1-x)U(x)SiO4 solid solutions.

Coffinite (USiO4), along with Th(1-x)U(x)SiO4 uranothorite solid solutions, are frequently present in reduced economically exploitable uranium ores. They could also control the concentration of uranium in the environment in the case of accidental release from underground radwaste repository. This paper reports for the first time a thorough FTIR and Raman study relative to the Th(1-x)U(x)SiO4 system, including synthetic analogues of thorite and coffinite end-members. Both sets of spectra confirmed the formulation of the samples and allowed to rule out the presence of structural water molecules and/or hydroxyl groups in the coffinite. Also, no characteristic signal of UO2(2+) uranyl ion was recorded, ensuring that uranium was fully incorporated under its tetravalent oxidation state. The variation of the positions corresponding to SiO4 internal vibration modes was then followed versus the chemical composition of the samples. If the FTIR spectra did not revealed any significant shift in the bands position, several Raman modes followed a linear trend as a function of the uranium incorporation rate. On this basis, Raman spectroscopy could be considered as a promising tool for the semi-quantitative determination of chemical composition of uranothorite samples, particularly for those coming from mineral ores. Finally, the data collected for the coffinite end-member, as the first to be obtained on pure synthetic samples, allowed a review of the results previously reported in the literature for this compound.

How to explain the difficulties in the coffinite synthesis from the study of uranothorite?

The preparation of Th(1-x)U(x)SiO(4) uranothorite solid solutions was successfully undertaken under hydrothermal conditions (T = 250 °C). From XRD and EDS characterization, the formation of a complete solid solution between x = 0 (thorite) and x = 0.8 was evidenced. Nevertheless, additional (Th,U)O(2) dioxide and amorphous silica were systematically observed for the highest uranium mole loadings. The influence of kinetics parameters was then studied to avoid the formation of such side products. The variation of the synthesis duration allowed us to point out the initial formation of oxide phases then their evolution to a silicate phase through a dissolution/precipitation process close to that already described as coffinitization. Also, the uranium mole loading initially considered was found to significantly influence the kinetics of reaction, as this latter strongly slows down for x > 0.3. Under these conditions, the difficulties frequently reported in the literature for the synthesis of pure USiO(4) coffinite were assigned to a kinetic hindering associated with the coffinitization reaction.